An electrocast shape, e.g., a brick is produced by application of the following basic steps:
a) melting a raw material such as Al.sub.2 O.sub.3 -ZrO.sub.2 -SiO.sub.2 ceramic or Al.sub.2 O.sub.3 -SiO.sub.2 ceramic by an electric furnace or other means of melting the raw material; PA1 b) introducing, e.g., pouring, the molten ceramic material into a mold; and PA1 c) cooling the molten ceramic material into a solid shape.
Conventionally, silica sand is used as a mold material, preferably silica sand of a relatively high purity mixed with an organic binder, e.g., a phenol or furan resin. Useful natural silica sands are Vietnamese Cam Lam silica sand and Australian Fremantle silica sand, both of which are readily available at reasonable cost. Silica sands having grain sizes of 45 mesh (Tyler Mesh Screen Size) or greater in size are normally used. The mold material (the mixture of silica sand and organic binder) is formed into a predetermined shape so as to make a mold. A box shaped mold, for example, may be made by first forming plates of the mold material, curing the binder and then assembling the plates into a box with the interior of the box forming the mold cavity. The brick shape is then formed from this cavity. Alternatively, for example, the mold may be formed as a single piece with an integral hollow cavity, corresponding to the brick shape formed therein. Both of these methods are well known to those skilled in the art.
After the mold is fabricated and, preferably, the binder has cured (so as to maintain the structural integrity of the mold), molten ceramic material is poured into the mold, followed by the gradual cooling of that molten ceramic material to solidification. Solidification progresses from the outside exterior surfaces to the inside or center of the brick (or other shape) until all of the molten material is solidified, a phenomenon also well known to those skilled in the art.
To ensure slow cooling, a thermal insulation layer is arranged outside the mold to cover the mold and the solidifying shape within. The material used for the thermal insulation layer may be silica sand, the same material used to form the mold, or granular alumina particle material (bauxite) or diatomaceous earth may be used. The mold itself also acts as an insulator to enhance slow cooling due to the proximity of the mold cavity surfaces to the solidifying ceramic material within.
As the molten ceramic material is poured into the mold, the mold absorbs heat from that molten material, firstly from the molten material proximate to the mold cavity surfaces. Thus, the surfaces of the casting are cooled first and solidification progresses from the outside to the inside of the casting. Just after pouring, the surface temperature of the molten material starts to decrease, while the temperature, of the molten material relatively remote from the mold surfaces, is still greater. This causes the surface of the casting to tend to solidify and shrink first, before those center portions of the casting which are relatively remote from the mold cavity surfaces. On the other hand the mold is concurrently heated by the hot casting, and it expands. Thus, the casting tends to shrink as the mold tends to expand. The surfaces of the mold cavity, which are dimensionally greater than other cavity surfaces, correspondingly expand to a greater degree, thus the mold and its cavity tend to deform by elongation. This tends to cause the corresponding surfaces of the solidifying casting to, likewise, elongate. This phenomenon is also well known to those skilled in the art. Those discrete sections of the casting which have semi-solidified and/or solidified, as the mold and its cavity surfaces are elongating, tend to be pulled apart. The result is that edge cracks and/or corner cracks are occasionally formed on the surfaces of such castings.
Where the casting and corresponding mold are of relatively large scale, the casting corners or shape transition areas tend to cool first, even before the other surfaces of the casting. Thus, those casting corners or shape transition areas tend to solidify first. In such a situation, as the mold tends to elongate, it is somewhat inhibited by the solidifying casting corners and shape transition areas. Also, the larger the mold, the less uniform is the temperature throughout the mold, thus the less uniform is the elongation. The result of these two factors is that the more expansive surfaces of the mold cavity tend to deform by warpage, usually producing a convex surface which, in turn, produces a concave surface in the corresponding adjacent surfaces of the casting. Therefore, where larger scale castings are to be made, the inside surfaces of the mold cavities are curved or concaved to compensate so that they will tend to flatten during deformation by elongation.
Silica sand has a relatively high rate of thermal conductivity, i.e., silica sand is a relatively good conductor of heat. It follows that conventional molds formed from silica sand have relatively good capacity for cooling castings. In addition endothermic reactions associated with the crystalline transformation of the silica (quartz) from .alpha.-phase to .beta.-phase to amorphous (glass) phase, as described hereinafter, increase the cooling capacity. On the other hand, the silica sand particles, themselves, expand during such crystalline transformations. This also tends to produce edge cracks and concave surfaces in the castings. Concave surfaces can sometimes be repaired by expensive diamond grinding. Edge cracks cannot be repaired, and sometimes cause the castings to fracture. It goes without saying that neither concave surfaces nor edge cracks are desirable.
The crystalline phase transformation of quartz in silica sand is a curious phenomenon. Natural silica sand is composed almost entirely of the .alpha.-phase quartz form of SiO.sub.2. When heated up to 573.degree. C., .alpha.-phase quartz transforms into .beta.-phase quartz (also known as cristobalite) with a transformation volume expansion of about 1.35%. At 1250.degree. C., .beta.-phase quartz transforms into quartz glass (also known as quartzite), the amorphous, non-crystalline, vitreous, fused form of quartz, with an ultimate transformation volume expansion of as much as 20% above about 1700.degree. C.
As mentioned before the silica sand mold is rapidly (almost instantaneously in respect to the mold cavity surfaces) heated by the introduction of hot, molten ceramic material being poured into the mold. The silica sand transforms and expands according to the degree to which each discrete particle is heated by the introduction of the molten ceramic material. Some particles may develop surface cracks while others may develop internal, grain boundary or intra-granular cracks. These particle may break apart or shatter. During heating, stresses are developed within the particle and the quartzite particles and fusions are quite friable as a result of the phase transformation and volume changes.
A mold needs permeability, i.e., the ability to transmit gases therethrough. Fine powder or dust diminishes the permeability of a mold, as the fine powders and/or dust tend to reduce the volume of space between the particles of silica sand used to form the mold. Therefore, when a mold material is to be reused, the fine powder and dust must be substantially eliminated. This fine powder and dust, created by the breaking up and shattering of the quartz particles during heat-induced phase transformation, are disposed of as industrial waste. It is desirable for considerations of economics and cost reduction that mold material should be reused as much as possible. But each time the mold material is reused, there is less available, as some is reduced to fine powder and dust. Indeed, in the case of the production of electrocast brick, the natural silica sand mold material can only be used about twice, on average, before it is reduced in size to such a degree that the cost of separating the fines and dust from the remaining particle exceeds the cost of using virgin material. Further, the generation and processing of the fine powder and dust, including its separation from the reusable particles, creates a substantially more adverse work environment and, thus, diminished labor efficiency.
It has been discovered that when molten ceramic material is introduced by pouring into conventional silica sand molds, some of the silica sand particles are instantly broken and shattered due to thermal shock, even before any crystalline phase transformation commences. The thermal shock is a result of radiation heating of the mold cavity surface sand particles produced by the adjacent proximity of the extremely high temperature molten ceramic material. These fragments separate from the surface of the mold cavity and are entrained in the molten ceramic material, flowing with that molten material as it migrates throughout the mold cavity. The crystalline structure of the quartz of these fragmented particles of silica sand, as explained above, phase transform to cause expansion within the solidifying casting, resulting in internal stress build-up and cracking. Some of these fractured silica sand particles, the finest of them, gasify to create voids and blow holes in the casting as it solidifies. Both types of defects essentially render the castings useless, thus producing scrap.
Attempts have been made to use alumina materials for the mold materials. Particles of tabular alumina and fused alumina, both of which are substantially the stable .alpha.-phase of alumina, have been tried. Also, attempts have been made to use alumina mixed with entrapped air to produce such molds, a technique normally used to produce light-weight, alumina bubble refractories for insulation purposes. But castings produced using alumina mold material become fused to the mold materials, and seized within the mold, as the molten ceramic material tends to weld itself to the cavity surfaces of the alumina molds. These seized mold materials can only be removed with the expenditure of great labor, at a prohibitive cost.
An object of the present invention is to provide a method for producing quality electrocast bricks and other ceramic shapes free from concave surfaces, cracks and blow holes. A further object of the present invention is to reduce mold cost by use of a reusable mold material. These and other objects of the present invention will become apparent from a reading of the following Summary of the Invention, Detailed Description and Claims.